1. Field of the Invention
The present invention relates to an optical device mounting apparatus and method for adjusting the angular position of the optical device without substantial linear displacement thereof and, more specifically, to an apparatus and method for enabling precise adjustment of the direction of a laser beam and/or precise translation without angular deviation.
2. Description of the Related Art
In various optical systems, manufacturers or operators must align and position multiple optical components with great accuracy and repeatability. Also, because lasers are often used as light sources in modern optical systems, it is often desirable to manipulate the direction of a laser beam (i.e., angular adjustment) without changing its linear position (i.e., without translation). Accordingly, it is often desirable to isolate angular adjustments of the optical system's components in each plane. Further, it is often desirable that angular adjustment of the components be independent from translational adjustments. All the final adjustments must then be ruggedly clamped to survive shipping, installation, and/or use of the product in the operator's environment. With conventional adjustable mounts, however, the act of clamping often disturbs the adjustment of the optical device, and poor clampability can result in angular displacement of the optical device during shipping and/or installation.
One such optical system that requires precision optical positioning of its components is a laser interferometer system. Laser interferometer positioning systems are key components in integrated circuit (IC) fabrication equipment, such as photolithography steppers, X-ray steppers, and E-beam machines. With the accuracy and repeatability available from laser interferometers, multiple layers on ICs can be precisely aligned, resulting in higher circuit densities and greater yields. IC inspection and repair systems also benefit from the precision of laser interferometer systems. A laser interferometer's measurement repeatability is crucial for manufacturing high storage capacity disk drives in the computer industry. Laser interferometer positioning is also used in the manufacture of other measurement devices, such as holographic, glass, and metal scales, and for calibration of the same. The high resolution and precision motion control capability of laser interferometers enable precision cutting machines (milling, turning, and grinding machines) to produce more accurate parts with smoother surface finishes. Laser interferometer positioning systems are also generally used in a wide range of other applications, including advanced metrology, coordinate measuring machines, mechanical vibration analysis, PC board manufacturing, and the like. Laser interferometer systems are especially useful in making linear distance, diagonal, and angular measurements, as well as for measuring flatness, way straightness, linear parallelism, and the like.
Laser interferometer systems are based on techniques pioneered by A. A. Michelson in the late 1890's to measure distances by counting wavelengths of the carrier signal. Michelson used a half-silvered mirror to split the beam from a light source into two beams, which were reflected from remote mirrors and again recombined at the half-silvered mirror. With the mirrors exactly aligned and motionless, the observer sees a constant intensity of light; but, if one of the mirrors is moved very slowly, the observer will see the beam repeatedly increasing and then decreasing in intensity as the light from the two paths adds and cancels. Each half-wavelength of mirror travel results in a total optical path change of one wavelength and one complete cycle of intensity change. If the wavelength of the light is known, then the travel of the mirror can be accurately determined.
To convert Michelson's apparatus into a crude electronic measuring instrument required only a photocell to convert beam intensity into a varying electrical signal, and an electronic counter to total the cycles of beam intensity. To make such a device practical, however, several other improvements were necessary. First, modern interferometers use lasers as light sources for two reasons: if the interferometer is to be used over any significant distance, the light must be pure (i.e., single wavelength). If the interferometer is to be accurate, then the wavelength must be exactly known. A second improvement included direction sensing electronics. A single photocell could not adequately sense the direction of movement of the target reflector. The method used by most conventional interferometers to sense direction includes splitting one of the optical beams into two portions, delaying one portion in phase by 90.degree., and then, after recombination, detecting each portion of the beam using a separate photocell. This technique produces two signals, which vary sinusoidally in intensity as the reflectors move, and the signals differ in phase brightness by 90.degree.. After DC amplification, these two signals can be used to drive a reversible counter, and the phase separation is sufficient to inform the counter of the direction of the motion. The third improvement, which has only partially been addressed, is precision positioning of the optical components (i.e., the mirrors).
Modern laser interferometer systems are commonly used by OEMS as the scale in precision motion control systems. Typically, these systems utilize several axes of linear and angular motion. Since one laser can supply light to six or more interferometer axes, there is a need for multiple beam splitters and beam bender mirrors to divide beams and deliver them to each interferometer. Also, since the output beam from an interferometer should be precisely parallel to the direction of axis motion, individual input beam legs must be angularly steered and translated in order to align to each interferometer.
The desire to adjust and fix the angular direction and position of laser beams with great accuracy and precision has increased in recent years. In various applications, such as medical, research, IC manufacturing, fiber optics and metrology, a long felt, yet unsolved, need exists to precisely adjust a laser beam's angular direction without affecting the beam's linear direction (i.e., without translating the beam). Also, the ability to precisely translate a laser beam without angular deviation has eluded artisans in the optics industry for years. These adjustments most often must be independent from one another and ruggedly clampable (as discussed above) without disturbing the initial alignment setting.
Conventional adjustable mounts are generally large, fragile, subject to vibration and thermal expansion errors, and are difficult to clamp. Furthermore, the act of clamping often disturbs the initial adjustment setting. Specifically, conventional kinematic mounts, such as the Newport.TM. Model 600A adjustable mount, typically offer good resolution but, in addition to the above, translate a laser beam while steering its angle. Therefore, these parameters (i.e., translation and angular position) cannot be independently adjusted. Gimballed mounts, such as the Newport.TM. SL and SK series adjustable mounts, typically avoid the beam translation problem by mounting the mirror to the base along a central axis of the mirror. However, these gimballed mounts exhibit poor resolution, environmental sensitivity, and clampability, along with fragility and large size. Semi-kinematic mounts, such as the Zygo.TM. Models 7010 and 7011 and the HP.RTM. Models 10710 and 10711 adjustable mounts, typically have almost unusable resolution and clampability for ultra-sensitive applications.
As the demand for more precise optical systems continues to grow, new and improved devices and methods for adjusting and fixing the angular position of an optical device and/or a laser beam are needed. To date, there is no efficacious and economically acceptable apparatus or method for performing this task.